What amateur operator running mobile has not lusted for
a bigger and better antenna? Well, so did I, and this is the
result...

Much taller than the average Ham
mobile antenna, this tree branch snapper tops out at 17 feet, 10 inches above
the road.

The antenna itself is 3.42
meters or 11 feet 2 inches long from the top of the mounting spring to the
tip.

In this picture, the 40 Meter
loading coil is in place.

A rear view of the antenna shows
the antenna with the shunt strap in place.

This strap is used for operation on
the 10, 12, and 15 Meter bands.

The 40 meter loading coil installed on the
antenna.

Yes, I do have to stop the vehicle and get out and change
loading coils when changing bands.

The original antenna was designed as a moderately
short antenna for the 10 meter band.

I originally wanted an antenna that would not hit
everything overhead while driving, since it was to be installed on a van. I
came up with an antenna that extended only 1.65 meters or about 5 feet 4 inches
above the top of the vehicle. It had a calculated efficiency of 42% when
compared to a half-wave dipole. It was my hope to be able to install a series
of different loading coils to be able to operate on the other HF bands.
However, calculations indicated that the antennas was simply too short to work
well as the frequency was lowered. Something better was needed.

The Evolution of
the Antenna Design

My first thought was to increase the length of the antenna below the
loading coil. My reasoning was that since that portion of the antenna carries
the highest RF current, then adding more length there would increase the
radiation efficiency of the antenna more than would adding the same length to
the antenna above the coil. Unfortunately, since the antenna is mounted on top
of a van which is a rather high vehicle, that would place the coil at a
dangerous height with regard to tree branches and the like. Since the loading
coils were going to be rather heavy due to their sturdy construction, placing a
heavy coil high up on a flexible support did not seem like a good idea. I
decided to investigate what increasing the length of the upper portion of the
antenna would do to the radiation efficiency.

It quickly became apparent that very good results could be obtained by
increasing the top whip length. As it turned out, this was due to two factors.
First, the antenna was center loaded, and second, increasing the top whip
length greatly reduced the coil losses due to the smaller number of turns
required. This also meant that I could use larger wire for the coils, which
further reduced the losses. It was almost like getting something for
nothing!

Since I had once a standard CB-length whip plus the magnetic mount and
base spring installed on the van, I knew that a total height of 3 meters was
workable, even though it did hit quite a number of overhead obstructions. I ran
calculations to determine how it would work. The results were very encouraging,
and I promptly set out to build the antenna. I still had the CB whip, and
figured that I could simply cut the top end off of it to get the total length
of 3 meters for the completed antenna.

While laying out all the parts on the ground to see how they would fit
together, I looked at the CB whip and realized that I would only have to cut
off about 17 inches. That seemed like a waste, and I really hated to cut that
small amount off of a perfectly good antenna.

Just for fun, I decided to repeat the calculations to see what would
happen to the gain if I left the extra 17 inches on the antenna. I was
surprised to find that the antenna efficiency increased between 19 and 31
percent, depending on frequency. The trade off, of course, was that I knew the
antenna was going to hit a lot more objects overhead than if I trimmed the 17
inches off of it. As a test, I assembled the antenna full length and drove
around with it for a few weeks to determined how much of a problem it would be.
I decided that I could live with it, as a necessary price for the increased
signal strength.

This table shows the results of the calculations. Notice the large jump
in gain between the original short antenna and the 3 meter antenna. Longer IS
better! <G>

I was able to make use of the basic antenna design and
most of the parts when constructed the final antenna. I'll show you in the
following pictures how I did it. This is a very easy antenna to build, and you
can do it with some simple tools and parts from the local home improvement or
hardware store.

When I made the 10 meter coil for the first 1.65 meter long
antenna, I used 1/4" OD copper tube for the coil. I inserted the ends of the
coil into a short length of 3/8" OD copper tube and then flattened the ends of
the tube. I then drilled a 1/4" diameter hole through the flattened ends so
they could be attached to the antenna posts. This coil should have had one more
turn on it. That way, the turns would have been spaced a bit further apart.
This would have lessened the coil losses somewhat. However, the calculated loss
in power was not enough to worry about, so I never bothered to make a new coil
for the antenna.

See that length of clear plastic between the ends of the
coil? When using a self-supporting coil like this one, you MUST use an
insulating brace like the plastic strip shown here or the white insulator
between the two antenna sections will fracture when you hit a big tree branch
with the antenna!

This is a close-up picture of one of the ends of the
loading coil.

You can see the coil tube inside the larger flattened
copper tube. The larger tube added structural strength to the ends of the coil
and provided a larger area for drilling the mounting bolt holes through the
coil ends.

A big antenna needs a serious mag-mount!

I purchased this magnetic mount for my mobile antenna years
ago over the Internet - I don't remember where I got it - but it holds to the
roof like a barnacle on a ships' hull. You can fold the antenna over until it
touches the roof of the van and the mag-mount stays put. It requires two hands
and a short pry-bar to get it loose.

See all that rust on the horizontal bar? That's from a
cheap steel quick disconnect I used for several years. It finally rusted to
then point that it made intermittent contact and I decided to get rid of it. So
far, I have not found anything strong enough to replace it. I have broken two
fairly heavy brass quick disconnects so far. So, for now, I just either tie the
antenna down to the roof of the van or unscrew it when I have to go into a low
garage. Needless to say, I strongly favor outside parking spaces!

This picture was taken before I installed the quick
disconnect. It shows the lower end of the bottom section of the antenna. The
antenna mast sections are constructed from hard wall copper pipe, 1/2" in
diameter. A brass plug with a 3/8" x 24 threaded hole was pressed into the end
of the copper pipe and hard-soldered into place. To hold the plug in place
during the soldering process, I rolled several grooves around the outside of
the copper pipe as seen here. I was careful not to roll them too deeply and
weaken the pipe.

A small "weep hole" is drilled through the pipe wall just
above the upper end of the brass plug that was soldered into the pipe. The hole
allows water to drain out of the antenna and not sit inside and corrode the
mounting bolt. Water WILL get into the antenna - you can't prevent it - so you
might as well make provisions to allow the water to drain out.

To hold the antenna to the base spring, I used a length of
threaded steel rod cut from a stainless steel bolt. After making sure that the
threaded rod was the the correct length and that everything fit properly, I
removed the mounting bolt from the base of the antenna and the base spring and
used thread locking compound (OK, I used Super Glue, if you must know) to
retain the bolt in the base of the antenna. That way I would not misplace the
bolt when I had to remove the antenna from the base spring.

The Center Insulator and Coil Mounts.

The lower section of the antenna is a 1/2" diameter copper
pipe about 60 cm (24 inches) long that extends upward to the lower coil
support.

The support itself is made from a 1/2" sweat "T" fitting, a
2" length of 1/2" copper pipe and a 1/2" sweat cap.

The bolts holding the coil in place are 1/4" diameter x
1-1/2" long stainless steel bolts. A 1/4" hole is drilled through the center of
each pipe cap before soldering it in place, and the bolts are temporarily held
in place by gently tightening the nut visible in this picture.

Each end of the antenna sections that thread into this
coupler has copper adapters that go from 1/2" OD copper pipe to 1/2" iron pipe.
These fittings have a male thread on them so they will screw into the plastic
coupler which then becomes the antenna center insulator.

Using plumbers hard solder (not rosin core radio solder)
and the proper cleaning flux, the copper components are soldered together.

Next, the interior of the copper sections is thoroughly
washed with clean water to remove any soldering flux residue and then placed in
the sun or some other warm place to dry.

When everything is dry, the nuts on the coil mounting bolts
are firmly tightened, and the sections may be screwed into the center
insulator.

Note that with this shunt strap in place, the plastic
strengthening strip is not needed. This is because the shunt strap is made from
a length of heavy silver-plated copper stock.

The top support for the loading coil is fabricated in the
same manner as the bottom support.

To hold the top whip, a 2" long length of 1/2" OD copper
pipe is soldered into the "T" fitting. Another female threaded brass plug is
inserted into the top end of the 2" long pipe section and soldered in
place.

Hey! What's that extra set of pipe fittings doing, and why
are they there? (Keep reading for the answer!)

A (very) close up photo of those extra pipe
fittings.

I always had a problem getting the antenna loading coil
lined up "just so" on the mount. When you screw the antenna into the mount, you
never know where the loading coil will be pointing when the mounting screw is
tight. With coaxial mounted loading coils, this is not a problem, but this
antenna has the loading coil mounted off-center from the mast. Provision needs
to be made to adjust the coil position. How come?

Well, the loading coil must always "trail" the antenna
mast; that is, the coil must be directly behind the antenna mast when the
vehicle is moving. The reason is rather obvious - after you hit the first big
low-hanging tree branch! If the coil happens to be in front of the antenna
mast, the coil may become snagged on the tree branch and instantly becomes part
of the local roadside litter. When the coil is mounted behind the mast, the
mast simply slides harmlessly beneath the tree branch, the coil does not get
hit, and all is well.

In order to accomplish this alignment without resorting to
the use of shim stock, various thicknesses of washers, and all sorts of other
chicanery, I decided I had to have a way to be able to rotate the coil around
the mast in some way. Since everything was soldered together, I came up with
the idea of using a pair of threaded mating fittings that I could simply twist
to get the alignment exactly correct. I drilled and threaded a pair of holes
through the outside (female) fitting so I could use a pair of stainless steel
screws to lock the fittings in place after after the adjustment was
complete.

The 40 Meter loading coil in place on the
antenna.

Notice the stainless steel nut at the bottom of the coil.
This nit, and another one at the top of the coil holds the coil on the antenna.
Changing coils is easy. Simply remove the nuts at the top and bottom of the
coil, swap coils, and replace the nuts. Tighten firmly - but not excessively -
and the PVC plastic acts as a lock nut to keep the coil in place. Still, it's a
good idea to carry a few spare nuts in the vehicle in case you drop one while
changing coils.

The complete Coil Set for the antenna.

From left to right -

Top Row: coil form made from 2" OD schedule 40 PVC pipe;
old self-supporting 10-meter coil made from 1/4" copper tubing; plastic support
spacer, used with self-supporting coils; copper shunt ring, used to tune
loading coils.

The 160 and 75-meter coils are wound using #14 AWG Nylon
insulated wire; all the rest of the coils are wound with #10 AWG THHN insulated
wire. Note that no terminals are used on the coils - the wire ends are simply
wrapped around the mounting bolts. When the coil is attached to the antenna,
the wire loops are pressed against the copper coil mounts to make the
electrical connection by the coil form when you tighten the 1/4" nuts from the
inside of the coil form.

General Coil Construction

The coil form is made from a length of 2" OD PVC pipe. I
cut each coil form about 3/4" longer on each end than the spacing between the
mounting bolts. NOTE: Make sure that each coil form fits easily over the
mounting bolts before winding on the wire or you'll have problems changing
coils.

After winding the wire on each coil, an application of
epoxy adhesive (J-B Kwik) was used to keep the coil turns in place. A coat of
black spray paint was applied for appearance and to make the white plastic coil
form less noticeable. NOTE: Apply the epoxy AFTER you have adjusted the coil to
resonance.

If you just wind the wire on the coil forms directly, this
will result in the coil "springing back" and becoming loose on the form when
you release the wire after winding it. I wound several extra turns on each coil
as I wound it on the form, then removed the coil from the coil form and then
squeezed it down around a slightly smaller form (actually a spray can of insect
repellent.) I then removed the now slightly smaller diameter coil and gently
screwed it onto the coil form, where it remained tight enough to stay in place
properly.

The 75-Meter coil

The calculations indicated that I would need to use #22 AWG
wire to get enough turns on the coil in the space I had available. The losses
would have been quite high, if I used that small size wire. So, this coil is
wound with #14 AWG wire. Since using larger wire would not allow as many turns
in the same length, I could either use a larger diameter form (too much wind
resistance and negative "eye appeal",) or a longer form, continuing the extra
turns below the lower mounting bolt (wind resistance, and higher RF losses.)
Something better was needed.

Remembering my switching power supply transformer winding
experience, I decided to try winding the loading coil for the antenna in the
same manner, that is, complete the first layer of the winding in the usual
manner, then "jump" the winding end back up towards the start of the first
layer and then continue the second layer of the winding towards the end of the
first layer of the winding. This is sometimes referred to as a "Z" winding.

You can see the black painted bolt in the right of the
photo that holds the end of the first layer of the winding. From that bolt, the
wire that begins the second layer "jumps" back to the left of the second layer
of the winding. The end of the second layer is brought out to one of the
mounting bolt holes. You can see the epoxy that holds the windings in
place.

The 160-Meter Coil

Wound in the same manner as the 75-meter coil, the
160-meter coil required only 60% of the number of turns as would be needed if
the windings were in a single layer. The trade off is that there are several
stray resonances of the antenna system using this coil, but none of them cause
any problems with normal operation. Since less wire is used in this coil than
would be used in a single layer coil, the copper losses are less, but the
dielectric losses are slightly higher due to the overlapped windings. However,
the use of the "Z" winding method minimizes the dielectric losses as much as
possible. The measured Rac loss of this coil is less than the originally
calculated coil using #22 AWG wire.

Tuning Ring in place inside the 40-Meter Coil

As it turned out, I didn't need to use it, but I found that
I could insert a shorted copper ring inside the coil and adjust the antenna
tuning plus or minus a turn or so on each coil. I also tried using various
types of ferrite and iron tuning slugs, but found that the copper ring produced
less extra loss in the coil than did the ferrite core. When the ring is placed
parallel to the turns on the coil, it acts like a shorted turn and reduces the
inductance of the coil. The ring fits in place by friction, and after adjusting
it, it may be permanently attached with some epoxy. Further minor adjustments
may then be made by bending the ring slightly.

ADDITIONAL
LOADING COIL DATA

( This loading coil data was added
on 24 July 2010 )

Since this article went on-line, I have received quite a few questions about
the exact construction of the loading coils. Although it is possible for a
careful observer to look at the photos posted on this page and deduce the
construction of the coils, it is probably a good idea for me to post a more
complete description of the construction of the coils so the reader can more
easily build them.

In the chart below, all the coils are wound on lengths of
Schedule 40 white PVC pipe. The actual end-to-end length of the coil windings
is shown in the chart. Due to the thickness of the insulation on the THHN wire,
the actual diameter of the finished coil will be close to 2.5 inches. The
inductance values are what I measured on my completed loading coils.

FREQUENCY BAND

INDUCTANCE

WIRE GAUGE

NUMBER OF TURNS

COIL LENGTH

COIL DIAMETER

COIL FORM

18 MHz

1.5 uHy

# 10 THHN

5

3.5 Inches

2.5 Inches

2" PVC sch 40 Pipe

14 MHz

3.8 uHy

# 10 THHN

9

3.5 Inches

2.5 Inches

2" PVC sch 40 Pipe

7 MHz

16.5 uHy

# 10 THHN

22

3.5 Inches

2.5 Inches

2" PVC sch 40 Pipe

4 MHz

55 uHy

# 14 THHN

Layer 1 = 34, Layer 2 = 6, overlaps layer 1

3.75 Inches

2.5 Inches

2" PVC sch 40 Pipe

2 MHz

100 uHy

# 14 THHN

Layer 1 = 34, Layer 2 = 20, overlaps layer 1

3.75 Inches

2.5 Inches

2" PVC sch 40 Pipe

Note that the coils for 160 and 75 meters have overlapping
coil windings. This can be avoided by using smaller diameter wire or using a
longer length coil form. Depending on where you place the top layer of wire on
the first layer (near one end or near the center of the first layer of wire)
the inductance of the coil will vary somewhat, and you may need to adjust the
number of turns on the coil. Tuning on the lower frequency bands will be more
critical, so you should expect to do some tuning as needed.

Please note that these exact coils may NOT work for you in
your particular situation. Factors such as whip
length, height above ground, size of the vehicle, etc., will require tuning the
antenna, either by tweaking the number of turns on the coils or adjusting the
length of the antenna's top whip slightly. In my case, "close enough" was good
enough, because I planned to use an antenna tuner in my mobile station. In any
case, the dimensions given in the chart above should get you "in the ball
park", as it were.

Tuning the
Antenna

A center loaded vertical antenna will not present a pure
resistive load at the base of the antenna. Usually, a matching network is added
at the bottom of the antenna to cancel the reactance and transform the lower
than 50 Ohm feedpoint resistance to something close to 50 Ohms. Since this
antenna was designed to operate over several HF bands, a single matching
network is impractical. Instead, I chose to connect the antenna through a
length of low-loss coaxial cable to an automatic antenna tuner (ATU) inside the
vehicle within reach of the operator and out of the weather.

To make the best use of the antenna with this set-up, the
loading coils for the antenna should be tuned to resonance at the high end of
each band. The antenna will then look electrically "short" to the tuner, which
will then be able to tune the antenna to the desired operating frequency. If
the loading coil in the antenna is tuned to a frequency below the top of the
band, then operation above that critical frequency will cause the antenna to
look electrically "long" to the RF. The loading coil will begin acting as a
choke and effectively reduce the length of the antenna, causing a severe loss
of gain.

Final Notes:

The top whip represents a (measured) capacity of 17.5 pF.
This value changes by about 0.75 pF as the mast is moved +/- 60 degrees from
vertical in any direction.

The tuning of the antenna stays fairly constant as the whip
sways, so compensation for bending of the antenna while driving is not
necessary, at least not on this vehicle.